Dimensionally reduction of linguistics information

ABSTRACT

A deep structured semantic module (DSSM) is described herein which uses a model that is discriminatively trained based on click-through data, e.g., such that a conditional likelihood of clicked documents, given respective queries, is maximized, and a condition likelihood of non-clicked documents, given the queries, is reduced. In operation, after training is complete, the DSSM maps an input item into an output item expressed in a semantic space, using the trained model. To facilitate training and runtime operation, a dimensionality-reduction module (DRM) can reduce the dimensionality of the input item that is fed to the DSSM. A search engine may use the above-summarized functionality to convert a query and a plurality of documents into the common semantic space, and then determine the similarity between the query and documents in the semantic space. The search engine may then rank the documents based, at least in part, on the similarity measures.

BACKGROUND

A traditional search engine processes a query by directly comparingterms in the query with terms in documents. In some cases, however, aquery and a document use different words to express the same concept. Atraditional search engine may produce unsatisfactory search results inthese circumstances. A search engine may augment a query by findingsynonyms of the query terms and adding those synonyms to the query. Buteven this tactic may fail to uncover conceptual similarities between aquery and a document.

To address the above drawbacks, the research community has proposedsearch engines which project queries and documents to a semantic space,and then match the queries to the documents in that space, rather than(or in addition to) comparing the lexical “surface” form of the queriesand documents. For example, a search engine may use the well-knownLatent Semantic Analysis (LSA) technique to perform the above-describedkind of processing. More recently, the research community has proposedmodels that express deeper relationships within input information, e.g.,through the use of neural networks having plural hidden layers. Forexample, auto-encoders leverage deep learning to project linguisticitems into a semantic space. One approach trains these auto-encoders inan unsupervised manner, e.g., by generating model parameters thatoptimize the reconstruction of documents, that is, after those documentshave been converted into a semantic space.

The above-described latent analysis techniques have, in some cases,improved the quality of search results. Yet there remains room forfurther improvement in this field of research.

SUMMARY

A deep semantic structured module (DSSM) is described herein forprojecting an input item to an output item in a semantic space. Forexample, the input item may correspond to an input vector thatrepresents one or more words, while the output item may correspond to aconcept vector that expresses semantic information regarding theword(s). A training system produces the model using click-through data.More specifically, in one implementation, the training systemdiscriminatively trains the model using the click-through data such thata conditional likelihood of clicked documents, given respective queries,is maximized, and the conditional likelihood of non-clicked documents,given the respective queries, is reduced.

In one application, a search engine may use the DSSM to map a query intoa semantic space. The search engine may then compute the respectivesimilarities between the query and a plurality of documents, within thesame semantic space. Based on those similarity measures (along withother optional ranking factors), the search engine can identify a subsetof documents which are most relevant to the query, if any. The searchengine may offer satisfactory performance in view of the fact that itsruntime objective (of discriminating relevant documents from irrelevantdocuments) is aligned with the objective function that was used to trainthe DSSM's model.

According to one illustrative feature, the DSSM may be implemented as adeep neural network (DNN) having plural hidden layers. The modelcorresponds to parameter values which define how information ispropagated through the DNN.

According to another illustrative feature, a dimensionality-reductionmodule (DRM) may transform the input item into a lower-dimension item,and then submit the lower-dimension item to the DSSM. Thelower-dimension item may be expressed in a space having a much smallerdimensionality compared to the space associated with the original inputitem. In one implementation, the DRM can perform the transformingoperation by expressing the input item as a plurality of n-grams, andthen mapping the identified n-grams into a lower-dimension vector withinthe dimension-transformed space. According to one benefit, the trainingsystem can perform its training operation in a more efficient manner byacting on lower-dimension items, as opposed to operating on the originaluntransformed input items (which may have very high dimensionality).

According to another illustrative aspect, the DRM can be also used as astandalone component, that is, without subsequently feeding thelower-dimension items to the DSSM.

The above approach can be manifested in various types of systems,components, methods, computer readable storage media, data structures,graphical user interface presentations, articles of manufacture, and soon.

This Summary is provided to introduce a selection of concepts in asimplified form; these concepts are further described below in theDetailed Description. This Summary is not intended to identify keyfeatures or essential features of the claimed subject matter, nor is itintended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative environment in which a search engineprocesses queries submitted by users. The search engine performs thistask using a similarity determination system, which, in turn, uses adeep structured semantic model.

FIG. 2 shows one implementation of the environment of FIG. 1.

FIG. 3 shows one implementation of the similarity determination systemof FIG. 1. The similarity determination system may employ one or moreinstances of a projection module.

FIG. 4 shows one implementation of a projection module, for use in thesimilarity determination system of FIG. 3.

FIG. 5 shows a portion of a deep neural network provided by theprojection module of FIG. 4, e.g., showing the manner in which valuesassociated with a first layer contribute to values associated with asecond layer.

FIG. 6 shows one application of the similarity determination system ofFIG. 3.

FIG. 7 shows one implementation of a dimensionality-reduction module,which is a component of the similarity determination system of FIG. 3.

FIG. 8 shows another implementation of a dimensionality-reductionmodule.

FIG. 9 shows one implementation of a training system, also shown inFIG. 1. The training system produces a model for use by the similaritydetermination system.

FIG. 10 is a flowchart that shows one illustrative manner of operationof the training system of FIG. 9.

FIG. 11 is a flowchart that shows one illustrative manner of operationof the similarity determination system, which is a component thatinitially appears in FIG. 1.

FIG. 12 is a flowchart that shows one illustrative manner of operationof a projection module, which is a component that initially appears inFIG. 3.

FIG. 13 is a flowchart that shows one illustrative manner of operationof a dimensionality-reduction module, which is a component thatinitially appears in FIG. 3.

FIG. 14 shows illustrative computing functionality that can be used toimplement any aspect of the features shown in the foregoing drawings.

The same numbers are used throughout the disclosure and figures toreference like components and features. Series 100 numbers refer tofeatures originally found in FIG. 1, series 200 numbers refer tofeatures originally found in FIG. 2, series 300 numbers refer tofeatures originally found in FIG. 3, and so on.

DETAILED DESCRIPTION

This disclosure is organized as follows. Section A describesillustrative functionality for mapping input items (e.g., input vectorsrepresenting one or more words) into a semantic space, using a modelthat is discriminatively trained based on click-through data. Section Bsets forth illustrative methods which explain the operation of thefunctionality of Section A. Section C describes illustrative computingfunctionality that can be used to implement any aspect of the featuresdescribed in Sections A and B.

As a preliminary matter, some of the figures describe concepts in thecontext of one or more structural components, variously referred to asfunctionality, modules, features, elements, etc. The various componentsshown in the figures can be implemented in any manner by any physicaland tangible mechanisms, for instance, by software running on computerequipment, hardware (e.g., chip-implemented logic functionality), etc.,and/or any combination thereof. In one case, the illustrated separationof various components in the figures into distinct units may reflect theuse of corresponding distinct physical and tangible components in anactual implementation. Alternatively, or in addition, any singlecomponent illustrated in the figures may be implemented by plural actualphysical components. Alternatively, or in addition, the depiction of anytwo or more separate components in the figures may reflect differentfunctions performed by a single actual physical component. FIG. 14, tobe described in turn, provides additional details regarding oneillustrative physical implementation of the functions shown in thefigures.

Other figures describe the concepts in flowchart form. In this form,certain operations are described as constituting distinct blocksperformed in a certain order. Such implementations are illustrative andnon-limiting. Certain blocks described herein can be grouped togetherand performed in a single operation, certain blocks can be broken apartinto plural component blocks, and certain blocks can be performed in anorder that differs from that which is illustrated herein (including aparallel manner of performing the blocks). The blocks shown in theflowcharts can be implemented in any manner by any physical and tangiblemechanisms, for instance, by software running on computer equipment,hardware (e.g., chip-implemented logic functionality), etc., and/or anycombination thereof.

As to terminology, the phrase “configured to” encompasses any way thatany kind of physical and tangible functionality can be constructed toperform an identified operation. The functionality can be configured toperform an operation using, for instance, software running on computerequipment, hardware (e.g., chip-implemented logic functionality), etc.,and/or any combination thereof.

The term “logic” encompasses any physical and tangible functionality forperforming a task. For instance, each operation illustrated in theflowcharts corresponds to a logic component for performing thatoperation. An operation can be performed using, for instance, softwarerunning on computer equipment, hardware (e.g., chip-implemented logicfunctionality), etc., and/or any combination thereof. When implementedby computing equipment, a logic component represents an electricalcomponent that is a physical part of the computing system, howeverimplemented.

The phrase “means for” in the claims, if used, is intended to invoke theprovisions of 35 U.S.C. § 112, sixth paragraph. No other language, otherthan this specific phrase, is intended to invoke the provisions of thatportion of the statute.

The following explanation may identify one or more features as“optional.” This type of statement is not to be interpreted as anexhaustive indication of features that may be considered optional; thatis, other features can be considered as optional, although not expresslyidentified in the text. Finally, the terms “exemplary” or “illustrative”refer to one implementation among potentially many implementations.

A. Illustrative Functionality

A.1. Overview

FIG. 1 shows an environment 102 that includes a training system 104 forproducing a deep structured semantic model 106 (henceforth, simply“model” 106). The training system 104 produces the model based onclick-through data maintained in a data store 108. More specifically, aswill be explained in greater detail below, the training system 104produces the model 106 such that a conditional likelihood of clickeddocuments, given respective queries, is maximized, and the conditionallikelihood of non-clicked documents, for the respective queries, isreduced.

Different systems may make use of the model 106 that is produced by thetraining system 104. In the illustrative environment of FIG. 1, a searchengine 110 provides a search engine interface 112 for receiving a queryfrom a user, who inputs the query via a user device 114. A similaritydetermination system 116 then uses the model 106 to project the queryinto a semantic space. More formally stated, the similaritydetermination system 116 projects a query input item that represents thequery (e.g., corresponding to an input vector) into a query output itemin the semantic space. The query output item captures latent semanticcontent associated with the query, and may be expressed as a queryconcept vector.

The similarity determination system 116 can then match the query againsteach of a plurality of documents in the semantic space, to generate aplurality of similarity measures. More formally stated, the similaritydetermination system 116 determines the similarity, in the semanticspace, between the query output item and each document output item. Eachdocument output item, in turn, is produced by projecting a document(providing in a data store 118) into the semantic space, in the samemanner that the query is projected into the semantic space. Eachdocument output item may be expressed as a document concept vector.

A ranking module 120 may use the similarity measures, optionally alongwith any other features, to assign ranking scores to the documents,given the query. Each ranking score indicates the assessed relevance ofa particular document to the submitted query. Based on the rankingscores, the ranking module 120 can then generate a list of documentsthat are deemed most relevant to the user's query. The list may identifyzero, one or more documents. Finally, the search engine interface 112can return the list of documents to the user device 114 for the user'sinspection.

From a very high-level perspective, the model 106 is discriminativelytrained using an objective function that aims to boost the relevance ofclicked documents, and decrease the relevance of non-clicked documents,with respect to respective queries in the click-through data. Thisobjective function aligns with the principal objective of the searchengine 110 itself. For this reason, the search engine 110 may be able toleverage the model 106 to provide highly relevant search results toUsers.

In the case of FIG. 1, the user may specify a query in manual form,e.g., by typing one or more query terms into an interface page providedby the search engine interface 112. The user may also receive searchresults in a conventional manner, e.g., by receiving a ranked list ofhyperlinks and snippets associated with the identified documents.However, the search engine 110 can receive a search query in any manner,and provide its search results in any form. For example, in analternative case, the user device 114 can form a query by recognizingkeywords in a user's speech. In another alternative technique, somecomponent of the environment 102 can automatically formulate a querythat expresses features associated with a triggering circumstance. Forinstance, the user device 114 can formulate a query in an automaticmanner based the current location of the user.

Further, in the case of FIG. 1, the similarity determination system 116uses the model 106 to compare a query with at least one document, withina semantic space. But more generally, the similarity determinationsystem 116 can use the model 106 to determine the similarity between anytwo instances linguistic information, in any application context. Forexample, in another case, the similarity determination system 116 canuse the model 106 to compare two documents within the semantic space, orto compare two phrases within the semantic space, etc. For instance, auser may identify a first document in a corpus as a query document, andthen ask the similarity determination system 116 to identify one or moreother documents that are semantically similar to the query document, ifany.

Nevertheless, to simplify the explanation, the scenario most frequentlyevoked herein corresponds to the above-described case in which an enduser uses his or her user device 114 to submit a query to the searchengine 110. Further, to simplify the explanation, the similaritydetermination system 116 will henceforth be explained for the specificcase in which each input item constitutes an input vector and eachoutput item constitutes a concept vector. A vector, as that term is usedherein, broadly corresponds to any information having two or morecomponents, e.g., a component for each element in a given vocabulary orset.

FIG. 2 shows a system 202 which represents one implementation of theenvironment 102 shown in FIG. 1. Here, the search engine 110 isimplemented by one or more servers, optionally together with othercomputing equipment (e.g., data stores, routers, load balancers, etc.).The search engine 110 may be provided at one physical site ordistributed over plural physical sites.

Users operate respective user devices (e.g., user device 204 and userdevice 206) to interact with the search engine 110. Each user device mayrepresent any computing equipment, such as a personal computer, a laptopcomputer, a computer work station, a smartphone or other type ofcellular phone, a media consumption device, a tablet-type computingdevice, a wearable computing device, a set-top box, a game-playingconsole, and so on.

Each user device may interact with the search engine 110 via anycommunication mechanism 208, such as a wide area network (e.g., theInternet), a local area network, a point-to-point connection, and so on,or any combination thereof.

In an alternative implementation, one or more aspects of the searchengine 110 can be implemented by each user device. For example, a usercan interact with a local computing device to search a local database.In that context, the local computing device can implement all aspects ofthe search engine 110.

FIG. 3 shows one implementation of the similarity determination system116 of FIG. 1. The similarity determination system 116 is depicted asincluding two or more instances of a projection module, e.g., a firstprojection module 302 and a second projection module 304, etc. Eachinstance of this projection functionality maps an input vector, whichrepresents linguistic information, into an output concept vector. Theconcept vector is expressed in a semantic space and reveals semanticinformation regarding the linguistic information.

More specifically, in one case, the similarity determination system 116can actually provide two or more separate instances of projectionfunctionality, to implement the plural projection modules (302, 304)shown in FIG. 3. For example, the similarity determination system 116can provide two or more physical processing components associated withdifferent instances of projection functionality. Alternatively, or inaddition, the similarity determination system 116 can provide separatesoftware-implemented instances of projection functionality, such asseparate threads, objects, etc. Alternatively, or in addition, thesimilarity determination system 116 can use a single instance of theprojection functionality to process separate instances of input items inseries, for example, by projecting a set of documents to the semanticdomain, one after the other.

Consider the first projection module 302. That functionality includes adimensionality-reduction module (DRM) 306 and a deep structured semanticmodule (DSSM) 308. The DRM 306 accepts an input vector which representslinguistic information. For example, assume that the linguisticinformation corresponds to a query that includes one or more strings(e.g., words). The original input vector can represent the linguisticinformation in any manner, such as a bag-of-words representation. Morespecifically, in that formulation, the input vector includes an entryfor each possible word in a vocabulary. Each entry has a value thatidentifies the number of times that the corresponding word appears inthe linguistic information. For example, for a query “choosing a dog atthe dog shelter,” the entry in the input vector for “dog” will beassigned a value of 2.

As set forth in Subsection A.3, the DRM 306 can apply any strategy totransform the input vector into a lower-dimension item. Thelower-dimension item may be expressed as a vector, and is henceforthreferred to as a lower-dimension vector. As the name suggestions, thelower-dimension vector has fewer dimensions compared to the originalinput vector. In other words, the space associated with thelower-dimension vector is smaller than the space associated with theoriginal input vector.

The projection module 302 reduces the dimensionality of each inputvector for the principal purpose of expediting the processing performedby the training system 104; otherwise, the training may becomepractically intractable in some cases. The use of the DRM 306 alsoexpedites the runtime processing of input vectors, e.g., correspondingto queries submitted by users, once the model has been generated in thetraining phase. In other cases, the training system 104 may performtraining on the “raw” input vectors, without reduction in the dimensionof the input vectors. For those cases, the projection module 302 canomit the DRM 306, in which case the DSSM 308 operates directly on theinput vectors in both the training phrase and the runtime applicationphase.

The DSSM 308 projects each lower-dimension vector (or the original inputvector) into a concept vector in a semantic space 310. The DSSM 308 usesthe model 106 to perform this task.

The second transformation module 304 performs the same operationsdescribed above on a second linguistic item. For example, assume thatthe second linguistic item corresponds to a document, or a portion ofthe document. A DRM 312 then transforms an input vector, whichrepresents the document, into a lower-dimension vector. The DSSM 314then projects the lower-dimension vector into a concept vector in thesemantic space 310. The DSSM 316 uses a model 106′ to perform this task.

In one case, the model 106 (used by the DSSM 308) is the same as themodel 106′ (used by the DSSM 314). The training system 104 may producethat single model in a single training process based on a single corpusof click-through data. In another case, the training system 104 producesthe model 106 in a first training process, and produces the model 106′in a second training process. Each training process can potentiallyoperate on a different corpus of click-through data. But in thefollowing explanation it will henceforth be assumed that a single model106 is used in all instantiations of the DSSM.

For simplicity, assume that symbol A represents a first concept vectorgenerated by the first projection module 302. The symbol B represents asecond concept vector generated by the second projection module 304. Asimilarity R(A, B) between A and B in the semantic space 310 representsthe extent to which the first linguistic information (e.g., the query)expresses similar concepts to the second linguistic information (e.g.,the document). The similarity between two concept vectors is inverselyproportional to the distance (d) between them in the semantic space 310,such that two concept vectors that are “close” together will have arelatively high similarity measure.

A similarity determination module 316 determines the above-describedsimilarity measure R(A, B). The similarity determination module 316 canuse any technique to make this computation, such as by forming a cosinesimilarity measure, as described in greater detail below.

A.2. The Deep Structured Semantic Module (DSSM)

FIG. 4 shows one implementation of the projection module 302 introducedwith respect to FIG. 3. To summarize, the projection module 302 includesa dimensionality-reduction module (DRM) 306 for mapping an input vector,here represented by x, into a lower-dimension vector. The input vectorrepresents linguistic information (such as a query or a document) as abag of words, or in some other manner of expression. A deep structuredsemantic module (DSSM) 308 projects the lower-dimension vector into anoutput concept vector within a semantic space. This subsection providesadditional details regarding the DSSM 308. The next subsection providesadditional details regarding the DRM 306.

The DSSM 308 may be implemented as a deep neural network (DNN), composedof a plurality of layers 402. FIG. 4 specifically shows that the DSSM308 includes four layers, but, more generally, the DSSM 308 can includeany number of layers. Each layer, in turn, includes a plural ofelements, referred to as neurons. Each neuron stores a value. Eachneuron, in a given layer, is furthermore connected to zero, one or moreneurons in an immediately anterior layer (if any), and zero, one or moreneurons in an immediately posterior layer (if any). Here, “anterior” and“posterior” refer to adjacent layers in relation to a direction ofinformation flow through the DNN, which is from bottom to top in FIG. 4.That is, with respect to a given layer, anterior layers represent lowerlayers, while posterior layers represent higher layers.

The layers 402 include a bottommost layer 404 for storing values,collectively denoted by z₁. More specifically, the layer 404 representsthe input to the DSSM 308, and therefore stores the values associatedwith the lower-dimension vector provided by the DRM 306. A next layer406 stores a collection of values z₂ that are derived from the values z₁in the first layer. A next layer 408 stores a collection of values z₃that are derived from the values z₂ in the layer 406. A final outputlayer 410 stores a collection of values y that are derived from thevalues z₃ in the layer 408.

FIG. 4 also represents the various layers (404, 406, 408, 410) as boxeshaving different respective lengths to indicate that the differentlayers have different respective dimensionalities. Without limitation,in one implementation, the input vector x fed to the DRM 306 has adimensionality of 500K, indicating that there are 500K possible words inan identified vocabulary. The layer 404 has a dimensionality of 30K. Forinstance, if an n-gram hashing technique is used to produce thelower-dimension vector, then the dimensionality of layer 404 means thatthere are 30K accepted n-grams (e.g., tri-grams) within the originalcorpus of 500K words. The layer 406 and the layer 408 each have adimensionality of 300 elements. And the layer 410 has a dimensionalityof 128 elements. Overall, the projection module maps the input vector(x), having a dimensionality of 500K, to the output concept vector (y)in the semantic space, having a dimensionality of 128 elements. Torepeat, however, another implementation of the DSSM 308 can employ anynumber of layers, each of which can have any size.

More formally stated, in one implementation, the values z₁ in the layer404 can be expressed as z₁=W₁x, where W₁ represents whatevertransformation is used by the DRM 306 to produce the lower-dimensionvector. The values z_(i) in layers 406 and 408 may be expressed asz_(i)=ƒ(W_(i)z_(i−1)+b_(i)), for i=2, . . . N−1. And the values y in thelayer 410 may be expressed as y=ƒ(W_(N)z_(N−1)+b_(N)), where, in thespecific example of FIG. 4, N=4. The symbol W_(i) denotes the i-thweighting matrix produced by the training system 104 (of FIG. 1), andthe symbol b_(i) refers to an optional i-th bias term, also produced bythe training system 104. The function ƒ(x), referred to as theactivation function, can be formulated in different ways, such as thefollowing tan h function:

$\begin{matrix}{{f(x)} = {\frac{1 - e^{{- 2}x}}{1 + e^{{- 2}x}}.}} & (1)\end{matrix}$

FIG. 5 clarifies the use the above equations with respect to smallportion of two layers, namely, a first layer 502 and a second layer 504.The individual elements in each layer correspond to neurons which storevalues. The value z₂₁ of the first neuron in the second layer 504 can becomputed by first forming a weighted linear combination of the values inthe first input layer 502 that may contribute to this value, as inγ=z₁₁*w₁₁+z₁₂*w₁₂+ . . . z_(1n)*w_(1n). The weighted sum γ can then beplugged into the above-stated tan h function (in place of x) to derivethe value for z₂₁. That is, z₂₁=ƒ(γ). The individual weighting valuesused in forming the weighted sum γ collectively form one of theweighting matrices W_(i) generated by the training system 104, e.g.,weighting matrix 506. For simplicity, the contribution of the biasfactors in forming the weighted sum has been omitted in the aboveexplanation.

FIG. 6 shows an application of the similarity determination system 116,and which also serves to summarize the components set forth with respectto FIGS. 3 and 4. In FIG. 6, a first projection module 602 produces aconcept vector which expresses a query (Q) in a semantic space. A secondprojection module 604 produces a concept vector which expresses adocument (D₁) in the same semantic space. A third projection module 606produces a concept vector which expresses a document (D_(n)) in the samesemantic space, and so on.

More specifically, the three (or more) instances of the projectionmodule can be implemented by separate physical components or softwareinstances. Or the three (or more) instances of the projection module canbe implemented by a single physical component or software instance,which processes linguistic items in series, one after the other.Further, the projection module 604 and the projection module 606, etc.can optionally perform their processing operations on the documents (D₁,. . . , D_(n)) as an offline process, that is, in advance of the usersubmitting the query Q.

A first similarity determination module 608 determines the similaritybetween the query Q and the document D₁ in the semantic space, while asecond similarity determination module 610 determines the similaritybetween the query Q and the document D_(n) in the semantic space. Asnoted above, each similarity determination module can compute thesimilarity between the query Q and a document D as a cosine similaritymeasure, as follows:

$\begin{matrix}{{R\left( {Q,D} \right)} = {{{cosine}\left( {y_{Q},y_{D}} \right)} = {\frac{y_{Q}^{T}y_{D}}{{y_{Q}}{y_{D}}}.}}} & (2)\end{matrix}$

The similarity measures, provided by the similarity determinationmodules (608, 610) constitute input features provided to the rankingmodule 120. The ranking module can rank the relevance of the documentsbased on the similarity measures alone, e.g., by identifying thedocument with the highest similarity measure as the most relevantdocument, and the document with the lowest similarity measure as theleast relevant document. Alternatively, the ranking module 120 can rankthe documents based on the similarity measures in combination with otherfeatures that have a bearing on the relevance of the documents. Forexample, the ranking module 120 may employ a ranking function that istrained by a machine learning technique based on any combination ofdescriptive features associated with the queries and documents.

A.3. The Dimensionality-Reduction Module (DRM)

FIG. 7 shows a dimensionality-reduction module (DRM) 702, whichrepresents one implementation of the dimensionality-reduction modules(306, 312) of FIG. 4. Recall that one purpose of the DRM 702 is toreduce the dimensionality of each input vector, so as to render trainingperformed by the training system 104 more efficient, and thus morecommercially feasible, and to expedite runtime processing of inputvectors in the application phase, e.g., in executing searches. Further,as will be described below, the DRM 702 can produce results that areuseful in other applications, that is, other than the scenario in whichDRM 702 serves as an input stage to a deep structured semantic module(DSSM).

To begin with, a transformation module 704 accepts an input vector andperforms one or more optional preliminary transformations on the inputvector. For example, assume that the linguistic information in questionis a query having the sole keyword “good.” The transformation module 704can modify the information associated with the input vector by adding abeginning token to the start of the word, and adding an ending token tothe end of the word, e.g., to produce “#good#”. This result is generallyreferred to as a converted item.

A partition module 706 breaks the converted item into a plurality ofn-grams. Each n-gram contains a sequence of n consecutive letters in theinput word. For example, in the case of FIG. 7, the partition module 706breaks the converted item “#good#” into the four tri-grams “#go,” “goo,”“ood,” and “od#”. A mapping module 708 then maps the identified n-gramsto entries in the lower-dimension vector. For example, the mappingmodule 708 can produce a lower-dimension vector having 30K entries,corresponding to all the possible permutations of three-letter sequencesthat may be used in the 500K vocabulary associated with the input space.All entries in the 30K-element vector will have a count of zero, exceptthe entries for “#go,” “goo,” “ood,” and “od#”, which will each have acount of 1.

There are relatively rare cases in which two different words may map tothe same n-gram vector. However, as these cases are very rare, the DRM702 can effectively ignore them without having a noticeable effect onthe performance of the DRM 702. Or the DRM 702 can provide specialprocessing to addresses these rare cases, e.g., by artificially inducingdistinguishing vectors for words which collide in the dimension-reducedspace.

FIG. 8 shows a dimensionality-reduction module (DRM) 802 having the samecomponents described above, that is, a transformation module 804, apartition module 806, and a mapping module 808. These modules performthe same functions that were described above with reference to FIG. 7.In the case of FIG. 8, however, the transformation module 804 performsany type of additional transformation on the input vector. For example,the transformation module 804 can first convert the input word “cat” toits phonetic representation, e.g., “kat”. The transformation module 803may then add beginning and ending tokens to the phonetic representationin the manner stated above, to produce “#kat#”.

As a point of clarification, FIGS. 7 and 8 represent cases in which eachinput vectors represent a single string. But an input vector may includetwo or more strings. In that case, the mapping modules (708, 808) canproduce counts that represent the total number of n-grams of particularkinds that appear within the two or more strings. For example, thephrase “good God” would include a count of 2 for the tri-grams “#go” and“od#” after ignoring the case. The transformation modules (704, 804) canoptionally join the two or more strings together before sending them tothe partition modules (706, 806), e.g., to create the single string“goodGod”.

Returning briefly to FIG. 3, the DRMs (306, 312) serve as input stagesto the DSSMs (308, 314). In an alternative case, the DRMs (306, 312) canfeed their lower-dimension vectors directly into the similaritydetermination module 316, without the use of the DSSMs (308, 314). Forexample, the first DRM 306 can form a dimension-transformedrepresentation of a query, while the second DRM 312 can form adimension-transformed representation of a word or phrase in a document.The similarity determination module 316 can then compare these twolower-dimension vectors to determine the similarity between them. Thismode of operation is potentially useful to help generalize therepresentations of instances of linguistic information, making them morelikely to agree despite the presence of small differences between thesurface representations of the linguistic items.

Finally, note that the DRMs (306, 312) in FIG. 3 can use any techniqueto reduce the dimensionality input vectors that are fed to them, ratherthan, or in addition to, the n-gram hashing technique described above.For example, in another case, the training system 104 can produce areduced-dimension matrix R having a prescribed distribution of values(but note that such training is outside the context of the training ofthe model 106). In one merely representative case, the matrix R can haveentries having values of 1 and −1 with equal probabilities, and valuesof 0 with a probability of 1−1/√{square root over (d)}, where d refersto the dimensionality of the space in which input information isexpressed. A DRM can then multiply the input vector x with the matrix R,as in z₁=Rx, to produce a lower-dimension vector. One drawback of thistechnique is that it does not, without modification, allow for theprocessing of newly-encountered words that are unaccounted for in theoriginal vocabulary. The n-gram hashing technique, on the other hand,can successfully address this situation because it can successfullyinterpret the n-grams in the new word.

The weighting matrix W₁ shown in FIG. 3 generically represents whateverinformation is used to by the DRMs (306, 312) to perform theirtransformation tasks.

A.4. The Training System

FIG. 9 shows one implementation of the training system 104 of FIG. 1.The training system 104 processes a corpus of click-through data(provided in a data store 108), to generate the model 106. The model 106represents the collection of weighting matrixes (W_(i)) and bias factors(b_(i)) shown in FIG. 3. Collectively, the parameter values associatedwith the model 106 are referred to using the symbol Λ.

The click-through data generally describes queries submitted by actualusers over some span of time, together with an indication of documentsthat the users clicked on and the documents that the users failed toclick on after submitting those queries. Here, the term “click” isintended to have broad connotation. It may describe the case in which auser literally clicks on an entry within search results, or some otherpresentation of options, using a mouse device. But the term click alsoencompasses the cases in which a user selects a document in any othermanner, such as by using a gesture on a touchscreen, a free-spacegesture that is detected using a video camera, a keyboard action, etc.In other cases, the term click describes the case in which a userexhibits interest in a document, without expressly selecting thedocument, such as by hovering over an entry associated with thedocument, or gazing at the document (which can be detected by eye gazerecognition technology), and so on. In other cases, some other entity,other than a single user, is the agent which performs the selecting,such as a crowd of users, an automated software agent (e.g., a BOT),etc. Likewise, the terms “query” and “document” have broad meaning, eachencompassing any linguistic information that includes one or morestrings.

However formed, the click-through data encompasses a plurality ofinstances of training data, each constituting a training example. Eachexample includes a query (Q), a document (D⁺) that the user selected inresponse to the query, and at least one document (D⁻) that the user didnot select in response to the query. In one case, a data collectionmodule can mine this information from archives of a search engine. Inthat situation, a non-clicked document (D⁻) of a training instance maycorrespond to an actual document that was offered to a user in responseto a query, but which the user declined to select. In other cases, thecollection module can randomly select any document from a corpus ofdocuments, so long as the user did not actually select that document inresponse to the query. But in this latter case, there is no requirementthat this document D⁻ was actually presented to the user. In any case,FIG. 9 shows that a training example includes four non-clicked documents{D₂ ⁻, D₃ ⁻, D₄ ⁻, D₅ ⁻}, but a training example can include any numberof such documents.

The training system 104 operates by using an iterative solving mechanism902 to iteratively achieve an objective defined an objective function904, by iteratively changing the parameter values of the model Λ. Whenthe iterative processing is finished, the final parameter valuesconstitute the trained model Λ. The objective function 904, in turn,mathematically expresses an aim which the model Λ seeks to achieve, whenfully trained. In the present case, the objective function stipulatesthat the conditional likelihood of the clicked documents, givenrespective queries, is to be maximized, and the conditional likelihoodof non-clicked documents, given the queries, is to be reduced. In otherwords, the objective function attempts to make the assessed relevance ofclicked documents as high as possible, while simultaneously attemptingto make the assessed relevance of non-clicked documents as low aspossible.

To mathematically derive the objective function, first note that theprobability P(D|Q) of a clicked document (D⁺) in a training example,given a query Q, can first be expressed as a softmax function asfollows:

$\begin{matrix}{{P\left( D^{+} \middle| Q \right)} = {\frac{\exp\left( {\psi\;{R_{\Lambda}\left( {Q,D^{+}} \right)}} \right)}{\sum_{D^{\prime} \in D}{\exp\left( {\psi\;{R_{\Lambda}\left( {Q,D^{\prime}} \right)}} \right)}}.}} & (3)\end{matrix}$

The term R_(Λ)(Q, D⁺) represents the similarity between the query Q andthe clicked document D⁺ in the semantic space, for a given currentselection of parameters associated with a model Λ. The term D representsthe set of five documents in the training instance, including D⁺ and thefour non-clicked documents, although, as stated above, a trainingexample can include any number of non-clicked documents. The termR_(Λ)(Q, D′) represents the similarity between the query Q and one ofthe documents (D′) in the training example, in the semantic space. Thesymbol ψ represents an empirically-derived smoothing factor (e.g., whichcan be generated by analyzing a held-out dataset of training examples).For this individual training example, the objection function willexpress an attempt to make R(Q, D⁺) as high as possible, and each R(Q,D⁻) as low as possible, to overall make P (D⁺|Q) as high as possible,and each P(D⁻|Q) as low as possible.

Overall, when considering all of the training instances in the corpus ofclick-through data, the objective function involves attempting tomaximize the conditional likelihood of the clicked documents given thecorresponding queries, or equivalently, minimize the following lossequation:

$\begin{matrix}{{L(\Lambda)} = {{- \log}\;{\prod\limits_{({Q,D^{+}})}{{P\left( D^{+} \middle| Q \right)}.}}}} & (4)\end{matrix}$

In other words, the objective is to derive a set of parameter valuesthat minimizes the above equation, with respect to all of the examplesin the empirical click-through data, or some subset thereof.

In one approach, the iterative solving mechanism 902 uses agradient-based numerical optimization algorithm to solve the aboveproblem, such as a gradient descent technique. The iterative solvingmechanism 902, for instance, can apply the following update rule toperform the optimization:

$\begin{matrix}{{{\Lambda_{t} = {\Lambda_{t - 1} - {\epsilon_{t}\frac{\partial{L(\Lambda)}}{\partial\Lambda}}}}}_{\Lambda = \Lambda_{t - 1}}.} & (5)\end{matrix}$

Here, Λ_(t) and Λ_(t−1) are the models at the t^(th) and t−1^(th)iteration, respectively, and ∈_(t) is a learning rate parameter. From ahigh-level perspective, the iterative solving mechanism 902 makes alarge change in the model Λ whenever the model Λ is changing by arelatively large amount from iteration to iteration, and makes a smallerchange in the model Λ whenever the model Λ is changing by a sloweramount.

More specifically, assume that there are M training examples in theclick-through data. The m^(th) training example is (Q_(m), D_(m) ⁺),corresponding to a particular pairing of a submitted query (Q_(m)) and adocument (D_(m) ⁺) that has been clicked in response to that query. Theloss function for that individual training instance is:L _(r)(Λ)=−log P(D _(m) ⁺ |Q _(m))  (6).

The derivative of the loss function as a whole can therefore beexpressed as:

$\begin{matrix}{\frac{\partial{L(\Lambda)}}{\partial\Lambda} = {\sum\limits_{m = 1}^{M}{\frac{\partial{L_{m}(\Lambda)}}{\partial\Lambda}.}}} & (7)\end{matrix}$

The term

$\frac{\partial{L_{m}(\Lambda)}}{\partial\Lambda}$can be derived as follows. To simplify the notation, the subscript mwill be omitted in the following. First, the loss function derived abovecan be expressed as follows:L(Λ)=log(1+Σ_(j)exp(−ψΔ_(j)))  (8),

where Δ_(j)=R(Q,D⁺)−R(Q, D_(j) ⁻). The gradient of the loss functionwith respect to the N^(th) weight matrix W_(N) is:

$\begin{matrix}{{\frac{\partial{L(\Lambda)}}{\partial W_{N}} = {\sum\limits_{j}{\alpha_{j}\frac{\partial\Delta_{j}}{\partial W_{N}}}}},{{where}\text{:}}} & (9) \\{{\frac{\partial\Delta_{j}}{\partial W_{N}} = {\frac{\partial{R\left( {Q,D^{+}} \right)}}{\partial W_{N}} - \frac{\partial{R\left( {Q,D_{j}^{-}} \right)}}{\partial W_{N}}}},{and}} & (10) \\{\alpha_{j} = {\frac{- {{\psi exp}\left( {{- \psi}\;\Delta_{j}} \right)}}{1 + {\sum_{j^{\prime}}{\exp\left( {- {\psi\Delta}_{j^{\prime}}} \right)}}}.}} & (11)\end{matrix}$

Now, let z_(i,Q) and z_(i,D) refer to the activation in the hidden layeri for query Q and document D, respectively. And let y_(Q) and y_(D)refer to the output activation for the output layer 410 for query Q anddocument D, respectively. Further, to simplify the following notation,let a, b, and c correspond to y_(Q) ^(T)y_(D), 1/∥y_(Q)∥, and 1/∥y_(D)∥,respectively. Finally, assume that the activation function used by themodel corresponds to the tan h function described above in Equation (1).With those definitions, each term in the right-hand side of Equation(10) can be calculated for the pair (Q, D) using the following formula:

$\begin{matrix}{{\frac{\partial{R\left( {Q,D} \right)}}{\partial W_{N}} = {{\frac{\partial}{\partial W_{N}}\frac{y_{Q}^{T}y_{D}}{{y_{Q}}{y_{D}}}} = {{\delta_{y_{Q}}^{({Q,D})}z_{{N - 1},Q}^{T}} + {\delta_{y_{D}}^{({Q,D})}z_{{N - 1},D}^{T}}}}},} & (12)\end{matrix}$

where:δ_(y) _(Q) ^((Q,D))=(1−y _(Q))∘(1+y _(Q))∘(bcy _(D) −acb ³ y _(Q))  (13)andδ_(y) _(D) ^((Q,D))=(1−y _(D))∘(1+y _(D))∘(bcy _(Q) −abc ³ y_(D))  (14).

In the above equations, the operation ∘ is an element-wisemultiplication (i.e., a Hadamard product).

The values {δ} for each successive hidden layer, moving down through theDNN in the direction of FIG. 3, can be computed through back projectionas follows:δ_(i,Q) ^((Q,D))=(1+z _(i,Q))∘(1−z _(i,Q))∘W _(i) ^(T)δ_(i+1,Q)^((Q,D))  (15)andδ_(i,D) ^((Q,D))=(1+z _(i,D))∘(1−z _(i,D))∘W _(i) ^(T)δ_(i+1,D)^((Q,D))  (16).

Correspondingly, the gradient of the loss function with respect to anintermediate weight matrix W_(i), i=2, . . . , N−1, can be computed as:

$\begin{matrix}{{\frac{\partial{L(\Lambda)}}{\partial W_{i}} = {\sum\limits_{j}{\alpha_{j}\frac{\partial\Delta_{j}}{\partial W_{i}}}}},{{where}\mspace{14mu}\frac{\partial\Delta_{j}}{\partial W_{i}}}} & (17)\end{matrix}$is equal to:(δ_(i,Q) ^((Q,D) ⁺ ⁾ z _(i−1,Q) ^(T)+δ_(i,D) ₊ ^((Q,D) ⁺ ⁾ z _(i−1,D) ₊^(T))−(δ_(i,Q) ^((Q,D) ^(j) ⁻ ⁾ z _(i−1,Q) ^(T)+δ_(i,D) _(j) ⁻ ^((Q,D)^(j) ⁻ ⁾ z _(i−1,D) _(j) ⁻ ^(T))  (18).

In a next “forward” phase of the training, the iterative solvingmechanism 902 uses the thus-calculated weighting matrices to reprocessthe various input vectors x, e.g., using the equations described inSubsection A.2. This operation updates the values associated with theneurons in the various layers 402 of the DNN. In a next “backward”phrase of the training, the iterative solving mechanism 902 then usesthe above equations to re-compute the weighting matrices. The iterativesolving mechanism 902 repeats the above-described training process untilthe DNN accurately models the behavior expressed in the click-throughdata, within a desired degree of tolerance. The bias factors b_(i) canbe derived using the same approach described above.

The training system 104 can perform the above-described processing usingparallel processing resources 906. The parallel processing resources 906can be implemented in any manner, e.g., using hardware units, softwareunits, or a combination thereof. For example, the training system 104can use an architecture similar to that shown in FIG. 3. Each separateprocessing unit, however implemented, can include adimensionality-reduction module (DRM) for calculating a lower-dimensionvector associated with a document D, a deep structured semantic module(DSSM) for generating an output concept vector y_(D) in the semanticspace, a similarity determination module for generating the similaritymeasure R(Q, D), and a softmax module for generating P(D|Q). Anotherprocessing unit can generate a representation y_(Q) of the query Q inthe semantic space.

B. Illustrative Processes

FIGS. 10-13 explain the various components of FIG. 1 in flowchart form.Since the principles underlying the operation of these components havealready been described in Section A, certain operations will beaddressed in summary fashion in this section.

Starting with FIG. 10, this figure shows a procedure 1002 by which thetraining system 104 produces the model 106. In block 1004, the trainingsystem 104 receives click-through data, e.g., from a search log archiveprovided by the search engine 110, or from some other source. In block1006, the training system 104 optionally transforms queries anddocuments in the click-through data into a dimension-reduced form usingone or more dimensionality-reduction modules (DRMs). This yieldslower-dimension items. In block 1008, the training system 104 generatesthe model 106 using the lower-dimension items (or the original inputitems). In one approach, the training system 104 performs this trainingsuch that the conditional likelihood of clicked documents, givenrespective queries, is maximized, and the conditional likelihood ofnon-clicked documents, given the queries, is reduced. In block 1010, thetraining system 104 stores the model 106. For instance, block 1010 mayconstitute installing the model 106 in the search engine 110.

FIG. 11 shows a procedure 1102 which describes the operation of thesearch engine 110 of FIG. 1. In block 1104, the similarity determinationsystem 116 receives a first input item (e.g., a first input vector)which represents first linguistic information, e.g., a query. In block1106, the similarity determination system 116 projects the first inputitem into a first output item (e.g., a first concept vector) expressedin a semantic space, using the model 106 trained as per the procedure ofFIG. 10. In block 1108, the similarity determination system 116 receivesa second output item (e.g., a second concept vector) that is alsoexpressed in the semantic space. For instance, the similaritydetermination system 116 may have previously produced the second outputitem by projecting a second input item, representing a document, intothe semantic space. In block 1110, the similarity determination system116 determines a similarity measure which reflects a similarity betweenthe first output item and the second output item, in the semantic space.The feedback loop indicates that the similarity determination system 116can perform the above-described operation for one or more additionaldocuments, to generate one or more additional similarity measures. Morespecifically, these per-document operations can be performed in series,as shown, or in parallel, or combination thereof. In block 1112, theranking module 120 ranks the documents based on the similarity measures,to produce search results. In block 1114, the search engine 110 outputssearch results that are provided in block 1112.

FIG. 12 shows a procedure 1202 that represents one manner of operationof the projection module 302 of FIG. 4. In block 1204, thedimensionality-reduction module (DRM) 306 receives an input item (e.g.,an input vector) that represents linguistic information, such as a queryor part of a document, etc. In block 1206, the DRM 306 transforms theinput item into a lower-dimension item, e.g., by performing n-gramhashing, random projection, or some other technique to produce alower-dimension vector. In block 1208, the deep-structured semanticmodule (DSSM) 308 projects, using the model 106, the lower-dimensionitem to an output item expressed in a semantic space. In block 1210, theDSSM 308 outputs the output item, e.g., by sending it to the similaritydetermination module 316 of FIG. 3.

FIG. 13 shows a procedure 1302 which explains one manner of operation ofthe DRM 306 of FIG. 4, which can be implemented using the functionalityof FIG. 7 or 8, or some other functionality. In block 1304, the DRM 306receives an input item that represents linguistic information, such as aquery or part of a document. In block 1306, the DRM 306 optionallyconverts the input item into a converted item, such as by addingbeginning and ending tokens to the word(s) associated with the inputitem, and/or converting the word(s) into their phonetic form. In block1308, the DRM 306 expresses the information associated with theconverted item as a plurality of n-grams. In block 1310, the DRM 306maps the identified n-grams to counts in a lower-dimension item.Overall, the procedure 1302 shown in FIG. 13 can be performed with orwithout subsequently processing the lower-dimension item using the DSSM308.

C. Representative Computing Functionality

FIG. 14 shows computing functionality 1402 that can be used to implementany aspect of the functionality of Sections A and B. For instance, thetype of computing functionality 1402 shown in FIG. 14 can be used toimplement the training system 104, and/or any component of the searchengine 110 (such as the similarity determination system 116), and/or theuser device 114, etc. In all cases, the computing functionality 1402represents one or more physical and tangible processing mechanisms.

The computing functionality 1402 can include one or more processingdevices 1404, such as one or more central processing units (CPUs),and/or one or more graphical processing units (GPUs), and so on.

The computing functionality 1402 can also include any storage resources1406 for storing any kind of information, such as code, settings, data,etc. Without limitation, for instance, the storage resources 1406 mayinclude any of RAM of any type(s), ROM of any type(s), flash devices,hard disks, optical disks, and so on. More generally, any storageresource can use any technology for storing information. Further, anystorage resource may provide volatile or non-volatile retention ofinformation. Further, any storage resource may represent a fixed orremoval component of the computing functionality 1402. The computingfunctionality 1402 may perform any of the functions described above whenthe processing devices 1404 carry out instructions stored in any storageresource or combination of storage resources.

As to terminology, any of the storage resources 1406, or any combinationof the storage resources 1406, may be regarded as a computer readablemedium. In many cases, a computer readable medium represents some formof physical and tangible entity. The term computer readable medium alsoencompasses propagated signals, e.g., transmitted or received viaphysical conduit and/or air or other wireless medium, etc. However, thespecific terms “computer readable storage medium” and “computer readablemedium device” expressly exclude propagated signals per se, whileincluding all other forms of computer readable media.

The computing functionality 1402 also includes one or more drivemechanisms 1408 for interacting with any storage resource, such as ahard disk drive mechanism, an optical disk drive mechanism, and so on.

The computing functionality 1402 also includes an input/output module1410 for receiving various inputs (via input devices 1412), and forproviding various outputs (via output devices 1414). Illustrative inputdevices include a keyboard device, a mouse input device, a touchscreeninput device, a digitizing pad, one or more video cameras, one or moredepth cameras, a free space gesture recognition mechanism, one or moremicrophones, a voice recognition mechanism, any movement detectionmechanisms (e.g., accelerometers, gyroscopes, etc.), and so on. Oneparticular output mechanism may include a presentation device 1416 andan associated graphical user interface (GUI) 1418. Other output devicesinclude a printer, a model-generating mechanism, a tactile outputmechanism, an archival mechanism (for storing output information), andso on. The computing functionality 1402 can also include one or morenetwork interfaces 1420 for exchanging data with other devices via oneor more communication conduits 1422. One or more communication buses1424 communicatively couple the above-described components together.

The communication conduit(s) 1422 can be implemented in any manner,e.g., by a local area network, a wide area network (e.g., the Internet),point-to-point connections, etc., or any combination thereof. Thecommunication conduit(s) 1422 can include any combination of hardwiredlinks, wireless links, routers, gateway functionality, name servers,etc., governed by any protocol or combination of protocols.

Alternatively, or in addition, any of the functions described in thepreceding sections can be performed, at least in part, by one or morehardware logic components. For example, without limitation, thecomputing functionality 1402 can be implemented using one or more of:Field-programmable Gate Arrays (FPGAs); Application-specific IntegratedCircuits (ASICs); Application-specific Standard Products (ASSPs);System-on-a-chip systems (SOCs); Complex Programmable Logic Devices(CPLDs), etc.

In closing, the functionality described herein can employ variousmechanisms to ensure the privacy of user data maintained by thefunctionality, if any. For example, the functionality can allow a userto expressly opt in to (and then expressly opt out of) the provisions ofthe functionality. The functionality can also provide suitable securitymechanisms to ensure the privacy of the user data (such asdata-sanitizing mechanisms, encryption mechanisms, password-protectionmechanisms, etc.).

Further, the description may have described various concepts in thecontext of illustrative challenges or problems. This manner ofexplanation does not constitute a representation that others haveappreciated and/or articulated the challenges or problems in the mannerspecified herein. Further, the claimed subject matter is not limited toimplementations that solve any or all of the noted challenges/problems.

More generally, although the subject matter has been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed above. Rather, the specific features and acts described aboveare disclosed as example forms of implementing the claims.

What is claimed is:
 1. A method performed by a computing device, themethod comprising: obtaining a query comprising one or more words from avocabulary having a first dimension; transforming the one or more wordsof the query into a phonetic representation of the one or more words;processing the phonetic representation to obtain a lower-dimensionrepresentation comprising a plurality of n-grams in an n-gram spacehaving a second dimension that is smaller than the first dimension;performing a natural language processing operation on thelower-dimension representation, the natural language processingoperation comprising determining similarity measures reflectingsimilarity of the one or more words of the query to a plurality ofdocuments; based at least on the similarity measures, selecting a subsetof the documents that are relevant to the query; and outputting theselected subset of documents in response to the query.
 2. The method ofclaim 1, further comprising: receiving the query from a user device overa network; and in response to the query, sending search resultsidentifying the selected subset of documents to the user device over thenetwork.
 3. The method of claim 1, further comprising: transforming thedocuments into other phonetic representations; and processing the otherphonetic representations to obtain other lower-dimension representationsof the documents, the natural language processing operation comprising:mapping the lower-dimension representation of the query and the otherlower-dimension representations of the documents into a semantic space;and determining the similarity measures based at least on relativedistances between the lower-dimension representation of the query andother lower-dimension representations of the documents in the semanticspace.
 4. The method of claim 3, wherein the one or more words areobtained as a bag-of-words representation of the query.
 5. The method ofclaim 3, further comprising: training a semantic model to map thelower-dimension representation of the query and the otherlower-dimension representations of the documents into the semantic spaceusing click-through data reflecting user selections of various documentsafter submitting various queries.
 6. The method of claim 1, wherein theprocessing comprises adding tokens to the phonetic representation,individual n-grams of the lower-dimension representation comprisingindividual tokens.
 7. A system comprising: a processing device; and acomputer readable storage medium storing instructions which, whenexecuted by the processing device, cause the processing device to:obtain a query comprising one or more words from a vocabulary having afirst dimension; transform the one or more words of the query into aphonetic representation of the query; process the phoneticrepresentation of the query to obtain a lower-dimension representationof the query, the lower-dimension representation comprising a pluralityof n-grams in an n-gram space having a second dimension that is smallerthan the first dimension; use the lower-dimension representation todetermine similarity measures reflecting similarity of the query to aplurality of documents; based at least on the similarity measures,select a subset of the documents that are relevant to the query; andoutput the selected subset of documents in response to the query.
 8. Thesystem of claim 7, wherein the instructions, when executed by theprocessing device, cause the processing device to: obtain click-throughdata reflecting user selections of various documents after submittingvarious queries; transform the various documents and the various queriesinto various phonetic representations; process the various phoneticrepresentations of the various documents and the various queries toobtain various lower-dimension representations comprising variousn-grams in the n-gram space; and train a semantic model using theclick-through data to map the various lower-dimension representationsinto a semantic space.
 9. The system of claim 8, wherein theinstructions, when executed by the processing device, cause theprocessing device to: transform the documents into other phoneticrepresentations of the documents; and process the other phoneticrepresentations of the documents to obtain other lower-dimensionrepresentations of the documents, map the lower-dimension representationof the query and the other lower-dimension representations of thedocuments into the semantic space using the semantic model; anddetermine the similarity measures based at least on relative distancesbetween the lower-dimension representation of the query and otherlower-dimension representations of the documents in the semantic space.10. The system of claim 9, wherein the instructions, when executed bythe processing device, cause the processing device to: receive the queryfrom a user device over a network; and in response to the query, sendsearch results identifying the selected subset of documents to the userdevice over the network.
 11. The system of claim 10, wherein theinstructions, when executed by the processing device, cause theprocessing device to: train the semantic model by iteratively adjustingmodel parameters of the semantic model to achieve an objective definedin an objective function.
 12. The system of claim 11, wherein theobjective function increases a conditional likelihood of clickeddocuments given respective queries.
 13. The system of claim 12, whereinthe objective function reduces a conditional likelihood of non-clickeddocuments given the respective queries.
 14. A computer readable storagemedium storing computer readable instructions which, when executed byone or more processing devices, cause the one or more processing devicesto perform acts comprising: obtaining a query comprising one or morewords from a vocabulary having a first dimension; transforming the queryinto a phonetic representation of query; processing the phoneticrepresentation to obtain a lower-dimension representation of the query,the lower-dimension representation comprising a plurality of n-grams inan n-gram space having a second dimension that is smaller than the firstdimension; using the lower-dimension representation of the query todetermine similarity measures reflecting similarity of the query to aplurality of documents; based at least on the similarity measures,selecting a subset of the documents that are relevant to the query; andoutputting the selected subset of documents in response to the query.15. The computer readable storage medium of claim 14, the acts furthercomprising: transforming the documents into other phoneticrepresentations of the documents; processing the other phoneticrepresentations to obtain other lower-dimension representations of thedocuments in the n-gram space; and using the other phoneticrepresentations of the documents with the lower-dimension representationof the query to determine the similarity measures.
 16. The computerreadable storage medium of claim 15, the acts further comprising:comparing the lower-dimension representation of the query directly tothe other lower-dimension representations of the documents to determinethe similarity measures.
 17. The computer readable storage medium ofclaim 15, the acts further comprising: adding a token to the beginningof the phonetic representation of the query to obtain thelower-dimension representation of the query; and adding the token to thebeginning of the other phonetic representations of the documents toobtain the other lower-dimension representations of the documents. 18.The computer readable storage medium of claim 17, the acts furthercomprising: adding the token to the end of the phonetic representationof the query to obtain the lower-dimension representation of the query;and adding the token to the end of the other phonetic representations ofthe documents to obtain the other lower-dimension representations of thedocuments.
 19. The computer readable storage medium of claim 18, thesimilarity measures being determined using the phonetic representationof the query having the token added to the beginning and the end of thephonetic representation, and the other phonetic representations of thedocuments having the token added to the beginning and the end of theother phonetic representations.
 20. The computer readable storage mediumof claim 19, the token being a character other than a letter.